1. Field of the Invention
The present invention relates generally to a position detector for a rotary element and more specifically to an improved arrangement wherein noise may be attenuated.
2. Description of the Prior Art
A previously proposed sensor arrangement is shown in FIGS. 1 and 2 of the drawings wherein a rotary element 1 is provided with a projection or marker 2 passing in close proximity of a stationary permanent magnet 3 about which a coil 4 is disposed. With this arrangement each time the marker 2 passes by the end of the permanent magnet 3, the change in magnetic flux induces an EMF in the coil 4. A typical output of the coil is depicted in FIG. 3. As shown in this figure the output takes the form of a sinusoidal wave having a peak value V.sub.p-p, which, as shown in FIG. 4, increases proportionally with the rotational speed of the member on which the marker 2 is formed. The output is also inversely proportional to the distance "1". So that the marker is effectively juxtaposed at the end of the magnet in essentially a single position, it is preferable to have the end of the magnetic disposed at a distance "L" from the surface of the rotary member. However, in the crowded environment in which an automotive transmission fly wheel (for example) is disposed (see FIG. 6) it is usually impossible to provide the required spacing "L" and/or to avoid other protrusions or voids on the rotary member such as holes, bolt heads, ribs and the like which are apt to produce noise such as indicated by "b" in FIGS. 3, 4 and 5. Hence, it is necessary in actual practice to use circuitry which screens or filters out the noise. However, with such circuitry, if as shown in FIG. 5, the slice level (En) is set sufficiently low (for example at E.sub.n1) to allow for positional detection at low RPM (e.g. N.sub.1 in FIG. 4), as the RPM rises (for example to N.sub.2 in FIG. 4) the level of the noise is apt to rise above the slice level, inducing erroneous position detection. Conversely, if the slice level is, as shown in FIG. 5, set high enough (for example at E.sub.n2) to avoid this phenomenon, then at low RPM the signal "a" is apt not to rise above the slice level adequately to permit accurate position detection.
In an attempt to solve this problem it has been proposed to replace the marker with a permanent magnet and use a soft iron core in place of the magnet in the stationary "pick-up" of the position detector. While this has to some degree overcome the above mentioned problem, the permanent magnet is subject to rapid loss of magnetism due to direct contact with the rotary member (usually made of ferromagnetic metal), exposure to heat, vibration etc. An example of an arrangement of this nature is disclosed in Japanese Utility Model Provisional Publication No. 51-85769/1976.